On some systems, the second `split-file` command will fail because the current
directory is the one we have deleted, we switch to another directory
before running it.
Previously, if a header was found via in a header map, and not just remapped.
we wouldn't also find the module it maps to when using implicit modules (for
module maps that were explicitly loaded).
This diff just updates these code paths to also locate the owning module via
`findUsableModuleForHeader`.
Reviewed By: benlangmuir
Differential Revision: https://reviews.llvm.org/D103930